1. Field of the Invention
The invention relates to a developing apparatus for developing an electrostatic image formed on an image bearing member with a developer, and more particularly to a developing apparatus using a mono-component developer and having a developer carrying member urged against an image bearing member.
Also, as the image bearing member, use can be made, for example, of an electrophotographic photosensitive member, an electrostatic recording dielectric member or the like, and the developing apparatus can be provided in a cartridge detachably mountable on an image reading apparatus (image forming apparatus) such as, for example, a copying machine or a printer, or the main body of the image forming apparatus.
2. Description of Related Background Art
For example, in an electrophotographic image forming apparatus, (1) a nonmagnetic contact developing method and (2) a magnetic non-contact developing method are widely used as conventional mono-component developing methods of developing an electrostatic latent image formed on an electrophotographic photosensitive member with a mono-component developer (Toner).
(1) Nonmagnetic Contact Developing Method
There has been proposed a method of carrying a nonmagnetic developer on a developing roller (developer carrying member) having a dielectric material layer, and bringing the developing roller into contact with the surface of a photosensitive member to thereby effect development (see, for example, Japanese Patent Application Laid-Open No. 2001-92201). The developer is a developing apparatus is supplied to the developing roller by a mechanical agitating mechanism or gravity. An elastic roller contacting with the developing roller is provided to thereby effect the conveyance and supply of the developer. This elastic roller also has the function of once removing any developer not shifted to the photosensitive member but residual on the developing roller, for the purpose of making the developer on the developing roller uniform. A DC bias is applied to between the base material of the photosensitive member and the developing roller.
(2) Magnetic Non-contact Developing Method
This method (see, for example, Japanese Patent Application Laid-Open No. S54-43027 and Japanese Patent Application Laid-Open No. S55-18656) uses a mono-component magnetic developer (magnetic mono-component developer), carries the developer on a developing sleeve (developer carrying member) containing a magnet therein, opposes the developing sleeve to a photosensitive member with a predetermined minute gap kept from the surface of the developing sleeve, and develops with the developer flying in this gap. The developer in a developing apparatus is conveyed to the developing sleeve by a mechanical agitating mechanism or gravity and also, the developer receives a contact magnetic force by the magnet and is supplied to the developing sleeve. Then, a predetermined developer layer is formed on the developing sleeve by regulating means, and is used for development. The force acting on the developer by the magnet is used not only for the conveyance of the developer, but also is positively used in a developing portion. In the developing portion, the developer is prevented from shifting to a non-image portion to thereby cause an image fault such as fog. That is, during development, the developer travels toward the magnet contained in the developing sleeve and receives the magnetic force thereof. For the flight of the developer, use is made of a bias comprising a DC bias and an AC bias superimposed thereon. The DC bias voltage is adjusted to a value between the image portion potential and non-image portion potential of the photosensitive member. Further, the AC voltage is superimposed and the developer is reciprocally moved to the image portion and the non-image portion, whereby the image portion is developed with the developer.
(3) Cleaner-Less (Toner Recycle) System
From the viewpoints of the simplification of an apparatus constructions and the elimination of waste, there has been proposed in an image forming apparatus of the transfer type an electrophotographic process of disusing an exclusive drum cleaner which is surface cleaning means for use after the transferring step of the photosensitive member, and recycling a developer in the apparatus. For example, there has been proposed an image forming apparatus which uses the aforedescribed nonmagnetic developing method to collect any developer not transferred and residual during development simultaneously with the development (see, for example, Japanese Patent Application Laid-Open No. H03-4276).
There has also been proposed an image forming apparatus which uses the aforedescribed magnetic non-contact developing method to collect any developer untransferred and residual during development simultaneously with the development (see, for example, Japanese Patent Application Laid-Open No. H10-307455) In the aforedescribed conventional nonmagnetic contact developing method mentioned under item (1) above, a reduction in fog performance has been a problem. The characteristic of the developer is reduced while the mechanical stripping-off by the elastic roller is repeated, and fog is sometimes aggravated by a reduction in the frictional charging characteristic of the developer or the like. Fog refers to an image fault in which the developer is slightly used for development in a white portion (unexposed portion) originally not printed and apparatus like a ground stain. For the prevention of the reduction in the characteristic of the developer, it is also possible to weaken the frictional contact force of the elastic roller, but the compatibility thereof with a ghost image fault is difficult. Here, the ghost image is the phenomenon that the hysteresis of a developer component used for development in the precious round of the developing roller apparatus as uneven density in a uniform halftone image with the phase difference of the outer periphery of the developing roller during the next and subsequent rounds. Also, the presence of a ghost means that there is some toner not stripped off but staying on the developing roller. That is, the toner is continuously subjected to the frictional contact by the elastic roller, and this is not preferable also form the viewpoint of the reduction in the characteristic of the developer. The adjustment of the frictional contact force is not only against the viewpoints of fog and ghost, but also is against the viewpoint of fog singly. Also, there has arisen the problem that when the characteristic of the developer is reduced, the developer is liable to be affected by the circulation thereof in the developing apparatus. Specifically, in a circulation using a mechanical force or gravity, there is formed an area in which particularly around the developing roller, the developer hardly changes places and is not circulated. On the other hand, the circulating developer suffers from a constant reduction in characteristic. Thus, the two kinds of developers, if mixed together when the developer in a container is decreased, has caused condensation or the like and has given use to a problem such as fog. Further, there is an image fault attributable to the elastic roller itself. As the elastic roller, use is made of one in the form of a sponge, form the viewpoint of the stripping-off and supplying performance for the developer, but the developer is compressed by the cells of this sponge and forms condensed lumps, and when these come out of the sponge to the surface, an image fault occurs particularly in a halftone.
On the other hand, in the magnetic non-contact developing method described under item (2) above, there is an image fault due to a magnetic ear. There is the problem that the uniformity of hair line differs between lengthwise and breadthwise. When the magnetic ear develops while moving in parallel to the movement direction of the photosensitive member (photosensitive drum), the uniformity of hair line is good, but in a direction orthogonal thereto, it becomes liable to break. Also, an image edge fault occurs. The edge of the high density portion, and particularly the downstream side of the process is darkly developed, and the edge of a halftone portion adjacent to the high density portion is lightly developed. The factor is expected to reside in developing while the developer is reciprocally moved in non-contact by an AC electric field. In a developing portion, the toner moves toward a surface, and the developer stagnates particularly downstream of an edge portion and conversely, the developer is drawn near from the exterior of the edge, thus causing such an image fault as described above.